cgroup: avoid false positive gcc-6 warning

arndb · htejun · commit cfe02a8a973e · 2016-03-16T13:32:23.000-07:00
When all subsystems are disabled, gcc notices that cgroup_subsys_enabled_key
is a zero-length array and that any access to it must be out of bounds:

In file included from ../include/linux/cgroup.h:19:0,
                 from ../kernel/cgroup.c:31:
../kernel/cgroup.c: In function 'cgroup_add_cftypes':
../kernel/cgroup.c:261:53: error: array subscript is above array bounds [-Werror=array-bounds]
  return static_key_enabled(cgroup_subsys_enabled_key[ssid]);
                            ~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~
../include/linux/jump_label.h:271:40: note: in definition of macro 'static_key_enabled'
  static_key_count((struct static_key *)x) &gt; 0;    \
                                        ^

We should never call the function in this particular case, so this is
not a bug. In order to silence the warning, this adds an explicit check
for the CGROUP_SUBSYS_COUNT==0 case.

Signed-off-by: Arnd Bergmann &lt;arnd@arndb.de&gt;
Signed-off-by: Tejun Heo &lt;tj@kernel.org&gt;
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
@@ -246,6 +246,9 @@ static int cgroup_addrm_files(struct cgroup_subsys_state *css,
  */
 static bool cgroup_ssid_enabled(int ssid)
 {
+	if (CGROUP_SUBSYS_COUNT == 0)
+		return false;
+
 	return static_key_enabled(cgroup_subsys_enabled_key[ssid]);
 }
 

Original file line number	Diff line number	Diff line change
`@@ -246,6 +246,9 @@ static int cgroup_addrm_files(struct cgroup_subsys_state *css,`
`246`	`246`	`*/`
`247`	`247`	`static bool cgroup_ssid_enabled(int ssid)`
`248`	`248`	`{`
	`249`	`+ if (CGROUP_SUBSYS_COUNT == 0)`
	`250`	`+ return false;`
	`251`	`+`
`249`	`252`	`return static_key_enabled(cgroup_subsys_enabled_key[ssid]);`
`250`	`253`	`}`
`251`	`254`